A conventional variable valve lift (VVL) mechanism to which the present disclosure is adapted is shown in FIGS. 1 and 2. The conventional VVL mechanism includes an inner arm 504 which has a rotatable roller 502 to come into contact with a low cam 500, causing a low lift action. A lost lever 510 covers the inner arm 504 and has a hinge shaft 506 as in the inner arm 504. The lost lever 510 is designed to come into contact with a high cam 508, causing a high lift action. A lock pin 512 moves back and forth in the inner arm 504 so as to switch an interlocked state between the lost lever 510 and the inner arm 504. A hydraulic lash adjuster (HLA) 515 is installed to support one side of the inner arm 504 and provides hydraulic pressure to the lock pin 512.
When an oil control valve (OCV) is turned on, the lock pin 512 is hydraulically actuated and extends outwards against an elastic force of a return spring 520 mounted in the inner arm 504. Thus, the lock pin 512 allows the lost lever 510 to move together with the inner arm 504 and increases a valve lift motion. When the OCV is turned off, the lock pin 512 is retracted into the inner arm 504 by the elastic force of the return spring 520 so as to nullify a movement of the lost lever 510, which affects the inner arm 504 The valve lift motion is relatively reduced by the inner arm 504 when the low cam 500 acts on the rotatable roller 502.
FIGS. 3 and 4 show on and off states, respectively, of a hydraulic circuit which includes an OCV 514 to actuate the lock pin 512. In the figures, when a solenoid of the OCV 514 is turned on, hydraulic pressure supplied from an oil gallery of a cylinder block is applied to the lock pin 512 so as to realize a high-lift state. In addition, when the OCV 514 is turned off, hydraulic pressure is not applied to the lock pin 512 so as to realize a low-lift state.
An orifice 516 in the hydraulic circuit secures a minimum hydraulic pressure in a deactivated state of the OCV 514 in order to maintain a function of the HLA 515 and to lubricate parts that are in contact with each other. The size of the orifice 516 is adjusted to secure the minimum hydraulic pressure.
However, the size of the orifice 516 cannot be actively regulated. Thus, if the size is relatively larger in order to secure a necessary minimum hydraulic pressure at a high-temperature idling state which is the least desirable hydraulic pressure-supplying state, a low-temperature middle-speed area which is the most desirable hydraulic pressure-supplying state essentially has a high hydraulic pressure even when the OCV 514 is in the off state.
In order to improve a problem of the high hydraulic pressure in the off state of the OCV 514, as shown in FIGS. 3 and 4, a relief valve 518 is additionally mounted to be connected to the hydraulic circuit only when the OCV 514 is in the off state so as to maintain a relatively low hydraulic pressure.
However, in very high-viscous oil conditions such as low temperatures including room temperature or when oil-degradation factors such as excessive soot in oil exist, oil discharge through the relief valve 518 is restricted. In these conditions, the lock pin 512 is inadvertently extended in the off state of the OCV 514, causing the high-lift state.
As conventional countermeasures for preventing such an inadvertent high-lift state, there is a method of increasing the elastic force of the return spring 520. However, it is difficult to switch to the high-lift state in the low-lift area, and the practical high-lift operation area is reduced, thus nullifying the application of the VVL.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.